Support additional read-only local repositories

Work is under way at Debian to support Maven as a tool for building packaged software.
Debian uses a local Maven repository (in /usr/share/maven-repo/) which is updated by the Debian tools (dpkg, apt-get). This repository is read-only for the normal user.

It would be great if users could use this repository in addition to their local repository in ~/.m2/repo, and it would simplify the Debian tooling.

In Debian, when building a package you cannot download anything from the Internet (to allow repeatable builds), so when packaging a Maven project, the --offline option is always used. That means that if you try to trick Maven and use file:///usr/share/maven-repo as a new repository, then it won't work as Maven believes that it is a remote repository.